The presence of moisture in gases leads to difficulties in many industries and operations. With a slight drop in temperature, condensation can occur in pipelines and reservoirs which can lead to corrosion, scales, freeze-ups, dirt, etc. which may damage instruments and controls and cause blockages in airlines, produce excessive pressure drops, increase down-time and reduce the life of tools. Similarly in chemical, food and metal working industries, the presence of moisture in the air and gases produces undesired oxidation. It has also been found that the robotics field requires extremely dry air for the operation of its pneumatic systems.
In order to produce extremely dry air i.e., dew points of minus 40 degrees Fahrenheit or lower, it is necessary to use an adsorptive drying system. Typically, adsorption drying is done by means of twin towers filled with a desiccant. The basis for the twin tower adsorption method is that while one tower is receiving moisture laden air for drying, the other tower is having its desiccant regenerated by passing dry air through that tower. In the past, this regeneration process has been accomplished by taking a portion of the dry air exiting the tower in the drying cycle and passing this air through the other tower to absorb moisture from the desiccant in that tower and thus regenerate that desiccant. Historically, a larger than desired percentage, typically 15 percent, of the dry air exiting the tower in the drying cycle was utilized to regenerate the other tower. This method has the obvious disadvantage of diverting 15 percent away from its end use in order to utilize it for regenerating one of the towers.
U.S. Pat. No. 4,738,692 issued Apr. 19, 1988 to Fresch, et al., discloses a pulse purge gas drying apparatus that markedly reduces the undesirably high percentage of dry air diverted to the regeneration tower. In the '692 patent, the gas drying apparatus includes purge outlet valves which release the air used for regeneration of the desiccant into the atmosphere, and controls for the purge outlet valves that alternately open and close or pulse the purge outlet valves during regeneration. As a result, the tower being regenerated is alternately pressurized and depressurized so that the amount of air released through the purged outlet valve is limited without causing a dew point elevation of the dry air. While prior regeneration methods used up to 15 percent of dry air generated by the drying tower, the apparatus in the '692 patent utilizes about 6 percent of the dry air generated by the drying tower.
In using the apparatus in the above described Fresch et al. patent, air flow was interrupted at the purge outlet valve, thereby allowing the regeneration tower to repressurize and hold for a predetermined time period of about 20 minutes, then depressurize after the holding time to resume purging that tower for about 20 minutes. This frequent depressurizing action creates purge loss and a momentary, extremely loud and annoying noise. In addition, when using a heater in the previously patented apparatus to accelerate regeneration of the desiccant, too much energy and time was expended in operating the heater. Subsequent cooling of the desiccant bed prior to switching the function of the towers resulted in further purge loss.
It is desirable to provide an improved heated, twin tower gas drying apparatus that rectifies the drawbacks of the prior systems by improving efficiency in purge loss and energy consumption while providing reduction in noise during operation.